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Original question:

DAC8760: DAC8760 Iout 4 ~ 20mA output question

DAC8760: DAC8760 Iout 4 ~ 20mA output question

Shinichi Inoue
Guru 20231 points
Part Number: DAC8760

Dear Specialists,

My customer is encountering the same problem.

Could you please see attached file below.

In his case, output current is 19.8mA when DAC8760 is set to 20mA.

According to the shread, the error comes from gain error, 0.21% x 0.016 =0.0336mA, it's far from 0.2mA 

The reverse current of Schottky diode and TVS diode is 0.01 uA.This is not still sufficient to 0.2mA.

What else can be considered as a factor of the error?

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Expert 5820 points

    Hi Shinichi,

    This is more error than I would expect from using the external transistor configuration. I do not see any apparent issues with your schematic that would lead to this error. What is the resistance of the load being driven? Also, what is the accuracy value of R21? This is a gain resistor for the current output stage and must be high precision to minimize output error. They could also easily switch to using the internal Rset and see if the error is eliminated.

    Thanks,

    Garrett

  • 0 in reply to Garrett Satterfield
    Guru 20231 points
    Hi Garrett,

    Thank you for your reply.

    I'll share your suggested point of the resistor accuracy(R21 and load resistor) with the customer.

    I'd like to confirm one point.
    According to the data sheet, the precision of Rset is stated only as Precision, but how much accuracy should it be?

    Also if he has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Expert 5820 points
    Hi Shinichi,

    Rset sets the reference current for the gain stage of the current output. The accuracy of this resistor will directly impact the accuracy of the current output so if the tolerance is 1% it could contribute this amount of error to the output. The selection of Rset depends on the accuracy required. I would recommend they look into using the internal Rset as resistors with very tight tolerances and low drift can be expensive.

    Let me know if they have any follow up questions.

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Guru 20231 points
    Hi Garrett,

    Thank you for your reply.

    I understand the resistance accuracy depend on the customer's system requirement.

    I'll share your suggestion with the customer.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    Guru 20231 points
    Hi Garrett,

    The customer confirmed internal Rset, the output current can be 20mA.
    It is expected.

    In the meantime, external Rset and the customer changed DVDD from DC/DC converter to internal supply.
    (DC/DC is removed and DVDD_EN is open.)
    At that time, It can also output 20mA.
    Regarding output current, is it possible to think the influence of DC/DCconverter?

    According to the datasheet P.52 10 Power Supply Recommendations, it is described about the influence of DC/DC converter.
    But I couldn't see about current output accuracy.

    I am looking forward to waiting your response.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Expert 5820 points
    Hi Shinichi,

    I would not expect that using the DC/DC for DVDD would cause an issue for the output accuracy as long as there are no large transients and decoupling capacitors are used. If there was switching noise coupling in, it would not result in a DC offset as they have observed but rather AC noise at the output.

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Guru 20231 points

    Hi Garrett,

    Thank you for your reply.

    I'll confirm DVDD decoupling capacitor and feedback.

    I appreciate your great help.

    Best regards,

    Shinichi